It is desirable that tools, such as are used on production lines, be permitted to be used until their useful life is ended, that is, until the work-piece which is being produced thereby no longer meets the production tolerances and quality control standards therefor. Once the tool has reached the end of its useful life, it should be changed and replaced by a new tool and, if feasible, the used tool can be re-sharpened for subsequent re-use.
Production costs can be substantially affected by the policy adopted by a manufacturer in changing tools as a result of wear. For example, if a tool is changed before it is worn, higher tool replacement costs and re-sharpening costs result. Further, costs are increased by the excessive down time caused by too frequent tool changes. On the other hand, if tools are not changed when they reach the end of their useful life, parts which no longer meet the production tolerances and quality control standards may be produced or an increased amount of tool breakage may occur.
In order to optimize a tool changing policy, it is desirable to devise a method which calls for an appropriate change of all tools before or as near as possible at the end of their useful lives. Methods which have been suggested up to now have not always been able to meet such a criterion or, in some cases, even to approach it effectively. Tool change policies up to now have often been based merely on the use of essentially fixed, periodic changes thereof, normally not determined by the actual wear of the tools. Thus, it has been suggested that some tools be changed regularly over a specified time period or after a selected number of work pieces have been produced thereby. However, due to a plurality of variables, such as the hardness of the work piece and the cutting conditions under which the tool operates, the life of a tool tends to be random in nature with standard deviations as large as one-half of the average values. As a result, under such procedures, many tools are changed before they reach the end of their useful life while other tools continue to be used beyond their useful lives.
Other procedures attempt to measure the wear characteristics of tools in some appropriate manner as by the use of indirect analyses of tool characteristics not specifically reflecting the actual wear characteristics of the tool itself. A summary of state-of-the-art techniques for sensing tool wear can be found in the publication, "Survey of the State of the Art of Tool Wear Sensing Techniques", by N. Cook et al., available from the Materials Processing Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, September 1975 and prepared under National Science Grant No. GI-43861.
The conclusion reached in the above survey is that present day techniques are inadequate to fill the needs of manufacturers, particularly where on-line techniques are required to obtain reliable tool wear information for tools having relatively complex geometries, such as for milling cutters and twist drills. The use of cutting process parameters, such as cutting forces, power consumption, cutting edge temperature, or machine vibrations, is unreliable because of the variations therein caused by variations in work-piece hardness and in cutting conditions which make it difficult to identify tool wear therefrom. Other more direct methods measure the dimensions of the wear zone in various manners using micrometers, profile tracers, weight measurements, and ultrasonic, optical or pneumatic techniques. Such methods prove difficult or impractical to implement in production line contexts and the accuracies thereof are often highly questionable. Moreover, since the dimensions of the wear zone are very small, it is difficult to obtain on-line measurements thereof in typical practical environments.
It is desirable that a reliable tool wear measurement technique be available and adaptable to production line use, the implementation thereof being capable of being embodied in a configuration which is compact, rigid and relatively insensitive to the environment and that such technique and apparatus be relatively easy to use and available at a reasonable cost.